Before you go for the Gladiator, find a weapon that can kill real fast, the best weapons are the Shotgun (which can be found in the middle of the Zeppelin), the K-SMG (Which can be found at the rear of the ship) and the Grenades (which can be found in one of the lower junction rooms between the large rooms). With those weapons, it'll be easy to take down the Gladiator and stay on top. Use the Grenades to clear away large groups of enemies and the Shotgun/K-SMG for individuals. Stay near a Grenade regeneration point and some health and you should have a Gold before the end of the required time.

An easier strategy is to make your way to the middle section, where there is a max damage powerup, kill the gladiator if you haven't already done so, and pick up the body armor on one of the ramps. Keep your pistols out and aim for the head, while constantly moving to evade enemy bullets. Keep picking up the max damage and armor as they spawn. If you die, just repeat the process and you should get gold or even platinum in no time.

A third strategy (and possibly more tedious than the other two) is to become gladiator, then pick up the Mag-Charger. While the gun has low damage, you can achieve kills from anywhere while shooting through any walls.